justaguy,

Since you haven't fertilized them yet and they are growing in vermiculite, they are probably suffering from one or more nutrient deficiencies. I recommend watering them with one-quarter strength nutrient solution. Avoid any formula that contains urea as a source of nitrogen.

Urea is fine in garden soil, where soil microorganisms break it down into usable nitrate or ammonium ions. In a sterile growing medium, urea doesn't break down or get used either, so it can build up to harmful levels.

I use three different urea-free soluble nutrient products:

Better-Gro Orchid Plus water soluble orchid food, 20-14-13 plus essential minor elements.

Better-Gro Orchid Bloom Booster, 11-35-15 plus essential minor elements.

Peters Specialties African Violet Plant Food, water soluble 12-36-14 with micronutrients

Any of those three formulas should work fine, although the first one is probably more suitable at this stage of growth. I get the Better-Gro products at Lowe's and the Peters African Violet Plant Food at Home Depot, our local greenhouse/garden center, and various other places.

None of those formulas contain calcium, so if you plan to grow those dahlia seedlings for any length of time in vermiculite, you should also supply some soluble calcium. I use calcium nitrate, which I purchase online at Everybody's Garden Center.

I dilute the calcium nitrate separately to about ¼ to ½ teaspoon per gallon and combine the diluted calcium nitrate solution with a similarly diluted nutrient formula. By mixing pre-diluted solutions, you prevent the calcium from precipitating out your soluble phosphates as relatively insoluble calcium phosphate.

MM

Thanks MM, I have just given them some fertilizer, but I wouldn't expect to see a nutrient deficiency develop with the seed leaves.

I am new to dahlia from seed, but don't most seeds contain enough nutrients for growth of at least the seed leaves?

At first glance it looked like sun scald to me.. as though you had gotten water on the leaves and perhaps they are too close to the light source.. or it could be tainted seed being this is only happening to your dahlias.

justaguy,

"...but I wouldn't expect to see a nutrient deficiency develop with the seed leaves."

Good point. I notice the leaf on the left edge of your photo also seems to be in some kind of distress that doesn't involve just the tip. Do you have any idea what the temperature is in the growing zone?

MM

The temps are approximately 65F in the air, as low as 60F overnight.

I will have to check when I get home to see if the 2nd set of true leaves is showing the same symptoms yet.

Yup the newest leaves are doing the same thing although not as bad (yet).

I have moved them to a different position under the light to see if too much light is the problem.

justaguy,

I notice a yellow cast to your photo, which suggests you are using a sodium light. I think cool white fluorescents are more suitable for seed starting, because of the higher blue spectral content. Would it be possible to try some cool white fluorescent light on at least some of your seedlings?

Also, I notice your vermiculite has a distinctive brown color to it. I know that some vermiculite has a tan rather than the usual gray color, but I never saw any look so very brown. Did something you watered it with turn it brown?

I don't know what the problem is at this point, but your dahlia leaves shouldn't be turning brown at the tips.

One of the reasons I don't like yellow sodium lights or purple gro-lites is the unnatural color they cast on the plants for photos, or for just visual observation.

MM

I am using a high pressure sodium bulb. While I agree it doesn't make for the most natural colored photos, it does the job for growing. A Metal Halide would be preferable for seed starting, but I can't justify replacing the HPS at this point.

Just going to wait and see if the plants outgrow whatever the issue is. The newest leaves appear to be doing better still so maybe whatever the issue is/was is lessening.